Molecular chaperones are a general term for proteins that form a complex temporally with client proteins to promote the formation of the conformation of the client proteins. These proteins, the activity of which is to help folding and association of protein and to prevent aggregation, are broadly defined as molecular chaperones and classified into several families according to their molecular weights (HSP90, HSP70, HSP60, HSP40, small HSPs and the like). In particular, HSP90 has been known to interact with many molecules which are involved in the intracellular signal transduction, and it is becoming clear that HSP90 is deeply involved in cell cycle regulation, and carcinogenesis, growth and survival signal of cells.
HSP90 is a molecular chaperone present in cells in abundance (occupies 1-2% of total soluble protein), distributed in the cytoplasm evenly and exists mainly as dimers. The activity of HSP90 alone in protein folding is low, and HSP90 functions cooperatively with other molecular chaperones having a folding activity (hereinafter called co-chaperones) such as HSP70 and p23. HSP90 is often needed for its function of client proteins that form a complex, and the action mechanism is based on the biochemical characteristic that HSP90 specifically recognizes a protein under the condition of unstable folding and binds to it. HSP90 performs ATP dependent folding (re-folding) of a denatured protein or a protein that is not folded. Especially, it is needed for constructing the structure of various key proteins (steroid receptors, Raf serine kinases, tyrosine kinases) which are involved in cancer related signal transduction. According to the recent findings, the control function of many key signal molecules is lost in human tumors, and these require HSP90 to maintain the function (non-patent document 1).
Geldanamycin (hereinafter called GM) is an ansamycin natural product, which was initially discovered in microorganisms as a tyrosine kinase inhibitor, but its direct inhibitory effect on a tyrosine kinase was low, and later it was found that this drug acted on HSP90 specifically. Radicicol (hereinafter called RD) is a macrolide natural product which, as a different structure from GM, also acts on HSP90 and inhibits its function. It has been known that GM and RD induce degradation of various key proteins (steroid receptor, Raf, Her2 and the like) which are involved in signal transduction related to cancer and cause growth inhibition of various cancer cells in vitro. HSP90 contains at the N-terminal an ATP/ADP binding site which plays an important role in controlling the chaperone function. This site is specific for and well preserved in the HSP90 family, and does not exist in other molecular chaperones. It has been elucidated by crystallographic analysis that GM and RD directly bind to this ATP/ADP binding site as antagonists (non-patent documents 2 and 3). It is also known that these antagonists inhibit the association with a co-chaperone such as p23 by binding to the ATP/ADP binding site. As the result, the composition of the chaperone complex which contains client proteins and HSP90 is changed, and eventually the client proteins are released from the complex and degraded mainly in the ubiquitin-proteasome pathway. Thus, the antiproliferative action on cancer cells by HSP90 antagonists will be caused by a depression of the client protein of HSP90 and the blocking of signal transduction pathway by the depression.
The HSP90 antagonist acts selectively on client proteins folded into HSP90, and does not affect the function and the amount of expression of other proteins at all. Studies have shown that in the process of carcinogenesis, a plurality of gene abnormalities are accumulated, and in many tumor cells, mutated proteins require more of the chaperone activity than normal proteins. HSP90 is overexpressed in various cancers. From the analyses of pharmacokinetics of a GM derivative, 17-AAG, in animal models, more of the 17-AAG is accumulated in cancer in comparison to the normal cells. From these reports, it is expected that the HSP90 antagonist acts on cancer cells specifically, not on normal cells. Also, since cancer cells under a kind of stressful condition such as abnormal protein expression, low oxygen and nutritional starvation are dependent on HSP90 at a higher degree, it would appear that the sensitivity of cancer cells against the HSP90 antagonist is higher.
Among the HSP90 antagonists, 17-AAG is subjected to ongoing Phase I/II clinical trials, and investigations on RD derivatives are also being conducted (non-patent document 4), but any one of these has problems for use as a pharmaceutical product in physical properties such as molecular weight, stability, and water solubility. A water soluble and low molecular weight HSP90 inhibitor is sought as a useful pharmaceutical product. An adenine derivative, PU3 and its derivatives have been reported to be a low molecular weight HSP90 inhibitor (Patent Document 1, non-patent document 5, non-patent document 6 and non-patent document 7). Also, a 1,3-dihydroxybenzene derivative, to which a 5-member ring is bound, has been reported to be a HSP90 inhibitor (patent document 2, patent document 3, patent document 4, patent document 5, patent document 6, and patent document 7), but the antiproliferative activity against cancer cells in vitro is weak (patent document 2). Further, the patent document 8 describes benzene derivatives, to which a 5-member ring is bound, as an antagonist of HSP90, but data of the HSP90 inhibitory activity of a derivative in which the 5-member ring has a triazole skeleton is not disclosed. On the other hand, the fact that the triazole derivatives of the present invention have HSP90 inhibitory activity is not known in literature.    Patent Document 1: International Publication No. 02/036075    Patent Document 2: International Publication No. 03/055860    Patent Document 3: International Publication No. 04/050087    Patent Document 4: International Publication No. 04/056782    Patent Document 5: International Publication No. 04/096212    Patent Document 6: International Publication No. 04/072051    Patent Document 7: International Publication No. 05/000300    Patent Document 8: International Publication No. 05/041879    Non-Patent Document 1: Hsp90 inhibitors as novel cancer chemotherapeutic agents. Trends Mol. Med. 2002; 8(4 Suppl.): p. S55-61.    Non-Patent Document 2: Inhibition of heat shock protein HSP90-pp 60v-src heteroprotein complex formation by benzoquinone ansamycins: essential role for stress proteins in oncogenic transformation. Proc Natl Acad Sci U.S.A. 1994; 91(18): p 8324-8328.    Non-Patent Document 3: Crystal structure of an Hsp90-geldanamycin complex: targeting of a protein chaperone by an antitumor agent. Cell 1997 Apr. 18; 89(2): p. 239-250.    Non-Patent Document 4: The clinical applications of heat shock protein inhibitors in cancer—present and future. Curr. Cancer Drug Targets. 2003 October; 3(5): p. 385-390.    Non-Patent Document 5: A small molecule designed to bind to the adenine nucleotide pocket of Hsp90 causes Her2 degradation and the growth arrest and differentiation of breast cancer cells. G. Chiosis et al., Chem. Biol. 2001 March; 8(3): p. 289-299.    Non-Patent Document 6: Targeting Wide-Range Oncogenic Transformation via PU24FCl, a specific Inhibitor of Tumor Hsp90. M. Vilenchik et al., Chem. Bio., 11, p. 787-797 (2004).    Non-Patent Document 7: Adenine derived inhibitors of the molecular chaperone HSP90-SAR explained through multiple X-ray structures. D. Dymock et al., Bioorg. Med. Chem. Lett., 14 (02), p. 325-328 (2004).